Various products have been developed for alerting the general public of the presence of abnormal situations, which sometimes require action to protect life and property. These products include broad area and local area applications. In local area products, the alerting task is usually performed by a visual and/or audible signal such as flashing or rotating lights and sirens. Local area products are most commonly used with emergency vehicles. Such systems are intended to warn the public in the proximity of the vehicles. Broad area products are exemplified by the United States Emergency Broadcast System, which uses broad area RF transmitters to communicate emergency signals over a large geographic area.
In general, the alerting products broadcast light, audio or radio frequency (RF) signals. Light and audio signals attenuate relatively rapidly as they propagate relative to RF signals. This property makes alerting products that broadcast light and/or audio signals particularly appropriate for alerting only over a range of several hundred yards from the site the broadcast originates--i.e., a local area. Also, light and audio signals are directly detectable by the human senses. On the other hand, RF signals are less attenuated by the atmosphere than light and audio signals and, therefore, they are effective for providing warning signals over a broad range of several miles or more, depending on the power of the signals at the broadcast site--i.e., a broad area. Moreover, RF signals pass through most building materials without substantial attenuation, making them particularly suitable for carrying warning signals inside a home or inside other environments that isolate occupants from ambient conditions.
Even though the range of RF signals is much more dynamic than light and audio signals, RF signals have the distinct disadvantage of requiring a recipient of the signals to possess a device for converting the RF signals to signals that one or more of the recipient's five senses can detect. Typical broad area, RF-based warning systems in the past have used conventional AM/FM receivers as the device for converting the RF signal. In these systems, a commercial broadcaster agrees to use its transmitter as part of a community warning system. The transmitters used by these commercial broadcasters are typically immobile, high power devices. In the United States, a national emergency network of this type is the Emergency Broadcast System. The network consists of a number of broad area broadcasters intended to blanket the United States with an emergency RF signal that can be received by conventional receivers. These emergency broadcasts using commercial-based broadcasting systems lack flexibility and are intended to provide blanket coverage over large geographic areas.
Traditional warning products that broadcast audible and/or visual warning signals are well suited for local area application. However, they have decreasing effectiveness when those intended to receive their signals are located in an insulated environment or an environment cluttered with other audible or visual "signals"--e.g., urban areas. For example, flashing lights are only perceptible within a direct line of sight and cannot be seen around corners of buildings, which is particularly a problem in urban areas. Likewise, warning sirens cannot be heard by individuals with reduced or impaired hearing or in noisy vehicles, urban streets or well-insulated dwellings. Other factors that reduce the effectiveness of local warning systems include the use of audio systems in a vehicle, heavy traffic conditions, weather conditions forcing windows to be rolled up and the noise of forced air systems. Also, vehicle and dwellings have recently become better insulated, further enhancing the masking of audio or visual warning signals. These signal masking problems increase the likelihood that an audible or visual warning device will not be noticed in a vehicle or building.
Moreover, even though local area warning devices are used by almost all emergency vehicles, such vehicles themselves often suffer from reduced awareness of other emergency situations. For example, a fire truck traveling in an emergency mode typically generates audible and visual signals to warn the motoring public and pedestrians that the truck is operating outside the normal mode of traffic flow (e.g., high speed and contrary to lane and signal flow controls). When two or more vehicles respond to an emergency from different points of origin, they typically approach the emergency along different routes thus becoming a risk to each other. In addition to the increased risk due to travelling in abnormal modes, the emergency warning siren or warning lights of a first vehicle tend to mask the ability to perceive warning lights or sirens of other emergency vehicles.
Various proposals to overcome these problems of traditional local area warning devices have been made. Several proposals have employed equipment dedicated to transmission of local area warning signals via RF transmission links. For example, known systems have utilized a dedicated transmitter located in an emergency vehicle and similarly dedicated receivers in vehicles or locations to be warned. These systems require the installation and maintenance of RF receivers in addition to conventional RF receivers of commercial bandwidths normally found in the vehicle or in the home and, therefore, these systems have failed to gain acceptance in the marketplace. Part of the absence of widespread use of such products relates to the perceived poor cost- to-benefit ratio for dedicated receivers that take up substantial space (especially in a vehicle) and are rarely used. Moreover, when they do operate, the user may view the conventional audible and visual warnings as effective alone and, therefore, conclude the dedicated receiver provides only an incremental and unnecessary improvement.
Another problem with known local area warning systems employing RF links is the practical difficulties related to installation of additional emergency radio equipment in a vehicle. Increasingly, mounting problems arise with the downsizing of vehicle passenger compartments coupled with limitations on mounting locations of such additional equipment due to interference with air bag deployment and the like.